Ocean Sunfish – Mola mola

Next on the list of showstoppers is one of the most bizarrely shaped fish in the sea, the Ocean sunfish!

The Ocean sunfish, also known as the Common sunfish or by its scientific name of Mola mola, is the second largest and best known member of the Molidea family. Closely related to pufferfish, porcupinefish, triggerfish and filefish, they are a small family with only five members, one of which was only recognised by science this week (stay tuned for an upcoming edition of the Findex!).

Ocean Sunfish

What’s in a name?

Mola mola have a few interesting names. They get their scientific name from the Latin word ‘mola’ meaning ‘millstone’ and from looking at them it is clear to see why. Their large, flat bodies resemble heavy millstones used to crush corn. The English name Sunfish comes from their habit of swimming horizontally close to the surface of the water, basking and sunbathing to warm themselves up. However, in most other languages they are referred to as Moonfish, again because of their appearance. Although most amusingly, in Germany they are also called Schwimmender Kopf which translates to swimming head.

Habitat and distribution.

Contrary to what many people think, Ocean sunfish can be found in most oceans around the world as they spend most of their time in deep, colder water. They are most often spotted in areas with high levels of nutrient upwelling, such as Indonesia and the Galapagos. These upwellings cause jellyfish, a Mola mola’s favourite food, to bloom in huge numbers. However, when the jellyfish numbers get high in the waters around the UK, Mola mola can often be spotted off the Welsh and Irish coasts so you don’t need to travel far to see one, provided the visibility plays ball!

Looks that kill.

One of the most visually unique fish in the sea, you can’t miss a Mola mola as it swims by! Round and flat, the name millstone makes a great deal of sense. They are usually a uniform grey in colour however they can be a much darker black, spotty or mottled. It is thought that there may be some region specific variations but this has not been confirmed yet. If you get close to them, you will see that they are often covered in parasites, usually some form of copepods. The reason that Mola mola are so susceptible to parasites is that they have no scales, which would normally prevent the copepods from being able to latch on so easily. This leads to some interesting behaviour we will talk about later.

Mola mola possess two small pectoral fins for stability but have very large, sail-like dorsal and anal fins which they use to propel themselves through the water. These fins have been often mistaken for shark fins as they swim close to the surface.

They do not have a caudal or tail fin. Instead they have a structure called the clavus which the fish uses as a rudder. It is made by the dorsal and anal fins growing and meeting together rather than being a deformed or vestigial tail.

Unlike most other bony fish, they do not possess a swim bladder. They maintain their body position by sculling their fins. They may also use liver oils to maintain buoyancy like a shark does but this is still unknown at this point.

Mola mola juveniles are also bizarre looking creatures that bear little similarity to their adult forms. They are almost perfectly spherical and orange but their skins surface is covered in translucent triangular spines. Although at just over 2mm across, they are a tricky spot in the open ocean!

Lifestyle choices.

Like a few other fish we have looked at in this series, Mola mola are broadcast spawners. This means that they release eggs and sperm into water for them to mix and drift away on the currents. Due to the range of the Mola mola being so vast, there are no areas that are known to draw them in to breed. Instead, they have been observed spawning in most of the oceans they are found in. Unlike many of the fish we have looked though, they do not change sex as they grow, although there are no visible differences between males and females underwater. The female Mola mola is thought to be one of the most fertile animals in the world, as when they breed they release over 300 million eggs. Unfortunately, they have a very low chance of survival but by releasing so many eggs at a time it gives their genes the best chance of surviving to the next generation.

Once the fry are born they shoal together for a while to increase their survival chances but as they grow they become more solitary. They are occasionally spotted in pairs or small groups but this tends to be in areas where jellyfish numbers are high.

Life on the reef.

Mola mola are most often observed by divers in surface waters, both in the blue and on reef environments. They like to swim horizontally close to the surface, warming themselves up in the sunlight. This allows them to spend longer in the colder water of the deep but also means that they can be more active in the cold than other predators.

The favourite food of a Sunfish is jellyfish, which they must consume in vast quantities as they are fairly void of nutrients. They also eat small squids and fish, crustaceans and sea grass.

Mola mola are the heaviest bony fish in the sea and one of the largest, growing to a maximum size of 3.3m in length and 4.2m from fin tip to fin tip. Most will grow to a weight of around 1000kg but some have been weighed at over 2300kg! This means that from fry to adult they will grow more than 60 million times their birth size, more than any other known vertebrate species! Due to their size they do not have many predators, however depending on where they are found they can be preyed upon by seals, orca, sharks and sea lions.

As mentioned before, Mola mola often are infested with parasites and this leads to some very interesting behaviours. They are frequent visitors to cleaning stations, providing a huge meal to any cleaner fish that are there. This works well for the smaller parasites but some can really dig in hard which calls for drastic measures!

Mola mola can sometimes be seen breaching the water before slamming down on one of their sides. This hopefully dislodges stubborn copepods and flushes the gills clear of nematodes, however very large fish tend not to breach as the shock of the landing can be fatal. Instead they employ seabirds to pick out the parasites for them. In a similar way to sunbathing, they float horizontally just under the water surface near the seabirds and wait to be cleaned. The strong beaks make short work of even the most determined parasite! Our budding marine biologists out there should keep their cameras poised to capture these behaviours as an airborne Sunfish is a rare shot and a great addition to a portfolio.

Personal experiences.

Myself and my colleagues have been incredibly lucky to see Mola mola around Indonesia but Bali seems to be the place where we have had the most luck. They are a curious fish, coming over to check divers out and swimming alongside us as we swim. Smaller fish may keep their distance so if they start swimming away it’s best to leave them to it. As I have mentioned in past editions, be careful taking lots of flash photography as they have no eyelids. Take one or two good ones rather than dozens to get one passable shot.

One thing you will notice about Mola mola is they are not slow moving fish. They appear lumbering and slow but they can swim faster than most divers and can swim in rapid bursts to escape or breach the water.

As I have also mentioned in other editions, do not touch a Mola mola! Their lack of scales means that their skin has no defence against the chemicals or bacteria that are on your hands. Even gloves will wipe away their defensive mucus, leaving them open to infections so HANDS OFF!

The bottom line.

We travelled to Bali to see these brilliant creatures and stayed at the Hai Tide resort. However, as I mentioned they can be seen throughout Indonesia as well as other seas. We also have great offers in Indonesia such as the Blue Manta Explorer, the WAOW and the Bunaken Oasis. We also have a great offer on for the Cyclone doing Wrecks and Reefs in the October half term. Mola mola have been seen there so make sure you bring your camera and check it out! Drop us a line to find out more!

I always get a shiver of excitement when I see a Mola mola. They are so strange but so interesting to watch swim around you, with their huge eyes staring back at you. Head out to the blue and go sunbathing with a Mola mola, you won’t regret it!

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